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Ichnology of a Pennsylvanian Equatorial Tidal Flat

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The Waverly Ichnofauna in Regional Context


The Waverly ichnofauna developed on a tidal flat connected with the open sea. Sedimentologic and ichnologic evidence from other exposures of the Stull Shale Member illustrate lateral variations in trace-fossil content and the environmental significance of the Waverly trace-fossil site in a broader regional context.

The Stull Shale Member crops out in a narrow belt across eastern Kansas. This unit contains numerous and excellent outcrops of tidal-flat deposits developed under contrasting paleoenvironmental conditions (Mángano and Buatois, 1997). Detailed stratigraphic sections were measured bed by bed, taking into account physical sedimentary structures, bed boundaries, geometry, and paleocurrents. Ichnologic information was added to data obtained from standard facies analysis. These tidal-flat deposits are stratigraphically equivalent, but they differ remarkably in trace-fossil content. Pennsylvanian successions in eastern Kansas are ideal for this type of study because horizontal, tectonically undisturbed strata can be followed for tens to hundreds of kilometers, allowing ichnologic comparisons along well-constrained time intervals. The Stull Shale Member is overlain by the Spring Branch Limestone Member, a laterally continuous transgressive unit that serves as a high-resolution marker bed. This situation allows comparisons with a degree of accuracy that usually is possible only in modern environments, but with the additional advantage of dealing with biogenic structures that already have passed through the taphonomic filter represented by the fossilization barrier. Sedimentologic and ichnologic information of different Stull Shale Member localities north of the Waverly tracefossil site is summarized below (fig. 74).

Figure 74--Location map of the sections of the Stull Shale Member studied in eastern Kansas. 1, Jackson Park quarry, Atchison County. 2, East of Perry Lake, Jefferson County. 3, Kansas Turnpike, Douglas County. 4, Between Kanwaka and Stull, Douglas County. 5, East of Lyndon, Osage County. 6, Waverly, Coffey County.

Sedimentology and Ichnology of Stull Shale Member Outcrops

East of Lyndon, Osage County, a 3-m-thick succession of the Stull Shale Member is exposed. At this locality the Stull Shale Member is dominated by fine-grained sandstones interbedded with thin mudstone partings. Soft-sediment deformation structures, including ball and pillow, pseudonodules, and convolute lamination, are dominant. Flat-topped ripples are present locally. Flaser and wavy bedding occur throughout the sequence. Intensity of bioturbation is low, and trace fossils are scarce, consisting almost exclusively of rare occurrences of the ichnogenus Trichichnus.

Excellent, laterally continuous outcrops of the Stull Shale Member, including its type section, are present in Douglas County. Most of these outcrops can be studied in roadcuts between the towns of Kanwaka and Stull, where this unit originally was defined. One of these outcrops was previously studied by Hakes (1976, his locality 8). Other outcrops are situated west/southwest of Clinton Lake, and along and adjacent to the Kansas Turnpike. Some of the latter also were described by Hakes (his localities 3, 5, 6, and 7). The Stull Shale Member is up to 9 m thick in this region. Sequences are dominated by thinly interbedded, very fine grained sandstones and mudstones displaying flaser, wavy, and lenticular bedding. Sand- and mud-filled channels usually cut the tidal-flat deposits (fig. 75A-B). Gutter casts are present locally. Soft-sediment deformation structures are abundant in the upper part of the unit at some localities. A thin coal bed occurs at the top of the Stull Shale Member at most of the outcrops. In terms of ichnofaunas, these outcrops contain a moderate to low diversity of biogenic structures. Teichichnus, Palaeophycus, Diplocraterion, Skolithos, Psammichnites, Lockeia, Nereites, Planolites, and Asteriacites are the most common traces. Hakes (1976) also mentioned Aulichnites and Chondrites in this area. Intensity of bioturbation is low to moderate locally. Typically, heterolithic facies are stacked forming fining-upward cycles that record tidal-flat progradation from sand- to mixed- and mud-flat environments (fig. 76).

Figure 75--Outcrop close to the town of Kanwaka. A. Photo. B. Drawing. Tidal-flat packages are cut by an intertidal runoff channel (arrows).

Figure 76--Outcrop close to the town of Stull. Parasequence of tidal-flat progradation, from sand- to mixed- and mud-flat deposits. Note Spring Branch Limestone Member at the top.

East of Perry Lake, in Jefferson County, the Stull Shale Member is up to 8 m thick and consists of flaser-, wavy-, and lenticular-bedded heterolithic facies. Gutter casts and sand-filled channels are common. Flat-topped ripples and convolute lamination are present locally. Poorly preserved plant fragments and carbonaceous debris increase in abundance toward the top of the section. Trace fossils are very rare and include only a few specimens of facies-crossing forms, such as Palaeophycus, and tiny specimens of bivalve traces (Lockeia, Protovirgularia). Intensity of bioturbation is low. Tidal-flat prograding successions are present.

Farther north, at the Jackson Park quarry, Atchison County, a complete sequence of the Stull Shale Member is exposed, with the Clay Creek Limestone Member and Spring Branch Limestone Member at base and top, respectively (fig. 77). At this locality, the Stull Shale Member is approximately 8.5 m thick and consists predominantly of massive siltstones. Very thin, current-ripple laminated, very fine grained sandstones occur locally. Plant fragments are abundant. Trace fossils are scarce and are restricted to the sandstone interbeds. Planolites and Palaeophycus are the only forms recognized, and degree of bioturbation is very low.

Figure 77--Outcrop at Jackson Park quarry. Both base and top of the Stull Shale Member are exposed. Clay Creek Limestone Member below, Spring Branch Limestone Member, above. Thickness of the Stull Shale Member is about 8.5 m.

Interpretation of Lateral Variability of the Stull Shale Member

Variation in trace-fossil types, ichnofossil diversity, burrow size, and degree of bioturbation reflect salinity gradients along an estuarine/embayment complex oriented northeast-southwest, with an open-marine system located to the southwest. In this model, outcrops in Atchison County represent the innermost facies of the embayment. In this area, fine-grained sediment accumulated on a mud flat close to the upper reaches of the estuary. Extreme fluctuations in salinity and temperature probably prevented the establishment of a significant benthic fauna. Planolites and Palaeophycus are very simple, facies-crossing forms that may be present in both marine and continental environments, and they therefore provide no clear evidence of marine influence.

In Jefferson County, tidal flats were most likely formed in an inner zone of the estuary bay (middle reaches). Low diversity of trace fossils indicates harsh conditions in a stressful environment, where extreme salinity fluctuations and freshwater influx make colonization by benthic fauna extremely difficult. However, presence of nuculoid bivalve traces indicates brackish-water conditions. Size reduction due to dwarfism is a common phenomenon in stressful, brackish-water ecosystems (Hakes, 1985).

Tidal-flat successions in Douglas County are inferred to have occurred in the middle zone of the bay, still under brackish-water conditions. Ichnodiversity, however, is higher than in sections located farther north. Hakes (1976, 1977, 1985) first noticed the brackish-water nature of the Stull Shale Member ichnofaunas in this area.

Pertinent features in the outcrop in Osage County also suggest a restricted setting with sand-bar and tidal-flat facies developing in the sand-dominated outer zone of the embayment system. Low diversity may reflect in part high hydrodynamic energy and extreme soft-sediment deformation.

When compared with the other Stull Shale Member localities, the Waverly trace-fossil site is remarkable in abundance and diversity of biogenic structures. This uniqueness does not seem to be associated with any significant change in lithology or physical sedimentary structures. Essentially the same lithofacies (heterolithic tidal-flat facies) occurs at all localities studied. Therefore, salinity may have been the master factor in Stull Shale Member trace-fossil distribution.

Brackish-water ichnofaunas typically display: (1) low ichnodiversity, (2) ichnotaxa commonly found in marine environments, but produced by euryhaline organisms, (3) dominance of infaunal traces rather than epifaunal trails, (4) simple structures produced by opportunistic trophic generalists, (5) combination of vertical and horizontal traces from the Skolithos and Cruziana ichnofacies, (6) presence of mono specific associations, (7) variable density, and (8) small size (Pemberton and Wightman, 1992; Mángano and Buatois, 1997).

In contrast to brackish-water assemblages, the Waverly tidal-flat ichnofauna is characterized by: (1) high ichnodiversity, (2) marine ichnotaxa produced by both euryhaline and stenohaline forms, (3) presence of both infaunal and epifaunal traces, (4) presence of simple and complex structures produced by trophic generalists and specialists (e.g., P. grumula), respectively, (5) dominance of horizontal traces of the Cruziana ichnofacies, (6) presence of multi specific associations, (7) high density, and (8) variable size. Additionally, the Waverly tidal-flat ichnofauna differs from freshwater tidal-flat assemblages of fluvio-estuarine transitions, such as those discussed by Buatois, Mángano, et al. (1997).

Pertinent features of the Waverly ichnofauna suggest that the biota inhabited a tidal flat dominated by normal-marine salinities connected directly to the open sea (i.e., outside of embayments). This interpretation is consistent with paleogeographic reconstructions showing restricted facies occurring toward the northeast and more open-marine facies occurring in the southwest, where Waverly is located.

Some workers suggested that deltaic systems were active during deposition of the Kanwaka Shale (Wanless et al., 1970; Hakes, 1976). In particular, Wanless et al. (1970) interpreted siliciclastic units of the Kanwaka Shale in Kansas as having been deposited in prodelta settings. However, our facies analysis suggests deposition in an embayment rather than in a deltaic setting. Wanless et al. (1970, fig. 5) suggested that prograding systems extended from the south, but ichnologic evidence observed in this study indicates more open-marine conditions toward the south.

Implications for Ichnofacies Models

Ichnofaunas from Tidal Successions in the Fossil Record

A review of the available information on ichnofaunas from tide-dominated successions allows us to understand the Waverly ichnofauna in a broader context and provides implications for trace-fossil facies models. For this review, we have selected a number of papers that integrate trace fossils and sedimentary facies.

A substantial amount of information is known about lower Paleozoic quartzites that commonly contain abundant ichnofaunas in subtidal-sandwave and intertidal-flat facies. For example, Baldwin (1977) documented several ichnotaxa from the Skolithos and Cruziana ichnofacies in tidal successions of the Cambrian-Ordovician of Spain. He showed that trilobite traces characterized beach and tidal-flat deposits, whereas vertical burrows of suspension feeders were abundant in barrier and subtidal sandstones. He concluded that the dominance of trilobite traces in onshore areas results from both actual abundance of tracemakers and enhanced preservational potential of the structures.

Mángano et al. (1996) analyzed Cambrian-Ordovician tidal siliciclastic rocks of northwest Argentina, formed in intertidal-flat and subtidal-sandwave environments. These authors noted that assemblages typical of the Cruziana ichnofacies occurred in protected settings landward of the high-energy, subtidal Skolithos ichnofacies. They pointed out that this resulted in the vertical replacement of the onshore Cruziana ichnofacies by the subtidal-sandwave Skolithos ichnofacies in a transgressive succession, and they urged caution in the application of the classical ichnofacies model of nearshore successions to tide-dominated settings. A similar pattern of trace-fossil distribution was recorded in other lower Paleozoic tide-dominated successions of northwest Argentina (e.g. Mángano and Buatois, 2000; Mángano et al., 2001).

Durand (1985) presented an exhaustive sedimentologic and ichnologic study of Ordovician tidalites of France. He identified a low-diversity assemblage dominated by suspension feeders in subtidal-sandwave facies and a Cruziana association in heterolithic facies of intertidal- to upper-subtidal origin.

Fillion and Pickerill (1990) documented in detail the trace fossil content of Cambrian-Ordovician siliciclastic rocks of Canada. They recorded 89 ichnospecies in the tidal-flat facies, representing a highly diverse occurrence of the Cruziana ichnofacies. In contrast, only two ichnotaxa (Skolithos and Diplocraterion) were found in subtidal deposits.

Legg (1985) documented sedimentary facies and ichnofaunas from a Cambrian tide-influenced delta system in Spain. He noted that Cruziana was more abundant in low-energy heterolithic facies of intertidal origin and that vertical equilibrium structures, such as Diplocraterion, showed a preference for high-energy conditions. Notably, he also found that Rusophycus tends to be more abundant in high-energy tidal channels.

Crimes et al. (1977) discussed the ichnology of Precambrian-Cambrian shallow-water successions in Spain. They noted the dominance of elements of the Skolithos ichnofacies in high-energy, thick sandstone packages, and more varied ichnofaunas in low-energy, thinly bedded heterolithic facies, characterized by the Cruziana ichnofacies.

Bjerstedt and Erickson (1989) analyzed Cambrian-Ordovician deposits of the northern United States and Canada. The Skolithos ichnofacies was present in high-energy, herringbone cross bedded sandstones formed in low-intertidal to subtidal settings. The Cruziana ichnofacies occurred in shallow and protected intertidal facies.

Poire and del Valle (1996) documented trace fossils from a sand wave complex in Cambrian-Ordovician rocks of Argentina. Ichnofossils were very rare or even absent in subtidal-bar deposits, but a relatively diverse suite of the Cruziana ichnofacies was present in interbar and bar-margin facies.

Stanley and Feldmann (1998) provided a very detailed study of Cambrian-Ordovician rocks in northern United States. In this case, the highest trace-fossil diversity occurred in the subtidal and lower-intertidal areas. Subtidal zones were characterized by quiet-water conditions, although periodically disturbed by storm action. Scarcity of biogenic structures in the upper-intertidal zone was related to preservational conditions.

Various sedimentologic studies of lower Paleozoic tidalites mentioned the associated biogenic structures (e.g., Thompson, 1975; Jansa, 1975; Barnes and Klein, 1975; Rust, 1977; Tankard and Hobday, 1977; Hiscott et al., 1984). Skolithos is the dominant, if not exclusive, component of trace-fossil assemblages in high-energy environments, such as tidal inlets, intertidal sandflats and channels, and subtidal sandwaves. In some cases, however, Skolithos is replaced by Diplocraterion (e.g., Cornish, 1986; Simpson, 1991). An assemblage of Arenicolites and Diplocraterion is present in the Cambrian Flathead Sandstone Formation of Wyoming, where the tracemakers colonized reactivation surfaces in these subtidal-sandwave quartzites (Boyd, 1966; Mángano and Buatois, personal observations).

A relatively diverse ichnofauna has been described from Silurian carbonate tidal deposits of Arctic Canada by Narbonne (1984). This intertidal association is dominated by domichnia and fodinichnia, with cubichnia and repichnia being less common. Although Narbonne (1984) considered this association analogous to the Skolithos ichnofacies, the high ichnodiversity, abundance of horizontal traces of deposit feeders, and variety of ethologic groups indicate that this intertidal assemblage belongs to the Cruziana ichnofacies. Low-energy, subtidal-shelf environments also are characterized by abundant and diverse trace fossils, including feeding, dwelling, locomotion, and resting traces, representing an example of the Cruziana ichnofacies (Narbonne, 1984).

Another set of data comes from the study of late Paleozoic tidal deposits. For example, Miller and Knox (1985) documented a diverse trace-fossil assemblage of the Cruziana ichnofacies in Pennsylvanian tidal-flat facies of Tennessee. This trace-fossil assemblage includes representatives of most ethologic categories, and although traces of suspension feeders are present, traces of deposit feeders represent the dominant trophic type.

Diemer and Bridge (1988) analyzed sedimentary facies of Carboniferous (Tournaisian) coastal deposits from Ireland. They mentioned a typical Cruziana assemblage in intertidal facies, with the highest diversity of trace fossils occurring in heterolithic tidal-flat facies.

Martino (1989) documented the ichnology of Pennsylvanian marginal-marine facies of West Virginia. In particular, biogenic structures are abundant and diverse in tidal-flat deposits. The assemblage was dominated by horizontal traces of deposit feeders and, to a lesser extent, grazers, reflecting a Cruziana ichnofacies.

High-diversity trace-fossil assemblages were recorded from Pennsylvanian tidal flats of Kentucky (Greb and Chesnut, 1994). This ichnofauna consisted of a mixture of vertical and horizontal traces, and included resting, dwelling, feeding, locomotion, and grazing traces. It is therefore considered to represent a Cruziana ichnofacies.

Tidal-flat trace fossils also were documented in Mississippian deposits in Illinois by Wescott and Utgaard (1987). This ichnofauna consisted of vertical traces of suspension and deposit feeders and horizontal traces of deposit feeders. Based on the dominance of vertical traces, these authors assigned this example to the Skolithos ichnofacies. However, the presence of horizontal feeding traces and trackways, existence of deposit feeders, ichnotaxonomic composition, and relatively high diversity observed in this study indicate instead that this assemblage is an example of the Cruziana ichnofacies that developed on intertidal flats.

Mesozoic and Cenozoic tidal ichnofaunas display similar onshore-offshore patterns to those of the Paleozoic. Triassic intertidal to supratidal deposits of British Columbia, described by Zonneveld et al. (1997) and Zonneveld et al. (2000), contain a low-diversity assemblage of feeding and dwelling traces. These authors suggested that extensive periods of exposure and fluctuations in salinity constrain development of the benthic fauna. Jurassic tidal-flat deposits of India also contain representatives of the Cruziana ichnofacies (Howard and Singh, 1985). The association includes dwelling, feeding, resting, and locomotion traces of deposit and, to a lesser extent, suspension feeders. The Alameda Avenue outcrop of Colorado is a well-known locality of the Cretaceous Dakota Group and has been described in a series of papers (e.g., MacKenzie, 1968, 1972; Weimer and Land, 1972; Chamberlain, 1980; Mángano and Buatois, personal observations). Tidal-flat deposits include abundant and varied biogenic structures, comprising both vertical traces of suspension feeders and horizontal traces of deposit feeders. Typical components are Teichichnus, Diplocraterion, Rhizocorallium, Chondrites, Arenicolites, Thalassinoides, Planolites, among many others, representing an example of the Cruziana ichnofacies. The Skolithos ichnofacies is present in higher-energy subtidal dunes and channels. Skolithos and Ophiomorpha are typical components of this assemblage.

Pollard et al. (1993) analyzed trace fossil evidence of colonization in Eocene sandwave deposits of England. This facies displayed a colonization ichnocoenosis containing Ophiomorpha and Macaronichnus, which occur along foresets and reactivation surfaces. Ophiomorpha seems to replace Skolithos as the dominant form of the Skolithos ichnofacies in post-Paleozoic, high-energy, shallow-marine sandstones (Droser and Bottjer, 1989).

Examples of Cruziana ichnocoenoses also are very common in modern tidal flats (e.g., Bajard, 1966; Howard and Dörjes, 1972; Swinbanks and Murray, 1981; Ghare and Badve, 1984; Frey, Howard, et al., 1987). Preservational potential of biogenic structures in intertidal settings is highly variable with a clear bias in the fossil record towards deeper-tier structures.

Ichnofacies Gradients in Tide- and Wave-dominated Shorelines

Nearshore trace fossils in wave-dominated settings have received a lot of attention. On the basis of the analysis of Mesozoic ichnofaunas from the North American and Canadian Western Interior Seaway, an ichnofacies model of nearshore deposits has emerged (e.g., Howard and Frey, 1984; Frey and Howard, 1985; MacEachern and Pemberton, 1992; Pemberton, Van Wagoner, et al., 1992). In particular, MacEachern and Pemberton (1992) proposed a model of onshore-offshore ichnofacies gradients that represents a refinement of the classical scheme of Seilacher (1967). In this model, four ichnofacies are distinguished: (1) the Psilonichnus ichnofacies in backshore areas, (2) the Skolithos ichnofacies in foreshore to middle-shoreface facies, (3) the Cruziana ichnofacies in lower-shoreface to offshore deposits, and (4) the Zoophycos ichnofacies in shelf zones. These authors also noted onshore-offshore trends in the trophic types involved. This model has been used successfully to delineate environmental zonations of nearshore siliciclastic rocks of different ages (e.g., Buatois et al., 1999).

However, this model applies only to the analysis of wave-dominated shorelines where there is a net increase in energy shoreward. This shoreward increase of energy parallels an increase in oxygenation, sand content, amount of organic particles in suspension, and mobility of the substrate. Nevertheless, this line of reasoning sometimes has been used erroneously in the analysis of ichnofaunas from tide-dominated environments. For example, studies of lower Paleozoic strata (e.g. Manca, 1986; Kumpa and Sanchez, 1988) commonly assumed a beach origin for large-scale, planar crossbedded quartzites with Skolithos, which were actually formed under subtidal conditions. In the same way, the presence of elements of the Cruziana ichnofacies in heterolithic facies commonly is regarded as evidence of subtidal conditions, although this is not necessarily the case.

Mángano et al. (1996) and Mángano and Buatois (1999) noted that the ichnofacies gradient in tide-dominated shallow seas is opposite to that in wave-dominated shallow-marine environments. As overall tidal energy increases from supratidal to subtidal settings, the Skolithos ichnofacies tends to occur seaward of the Cruziana ichnofacies. Analysis of the Waverly ichnofauna supports this conclusion. The Waverly assemblage is characterized by a mixture of horizontal, inclined, and vertical structures; dominance of crawling, feeding, and grazing structures of deposit feeders; high ichnodiversity and abundance; and presence of structures produced by mobile organisms. It therefore fulfills all the characteristics of the Cruziana ichnofacies (cf. Pemberton, MacEachern, et al., 1992).

Presence of the Cruziana ichnofacies in intertidal environments represents an occurrence in water shallower than expected according to standard ichnofacies models. This is the rule rather than the exception in tide-dominated settings. We therefore suggest that the classic onshore-offshore replacement model should be applied only in wave-dominated systems (fig. 78A) and that the opposite gradient is observed in tide-dominated systems (fig. 78B). This is consistent with information from modern tide-dominated environments, where the highest faunal diversity is present around mid-tide level (e.g., Beukema, 1976).

Figure 78--Trace-fossil facies models. A. Wave-dominated shoreline. B. Tide-dominated shoreline. Ichnofacies block diagrams based on Buatois et al. (2001). Skolithos ichnofacies: 1, Arenicolites. 2, Ophiomorpha. 3, Diplocraterion. 4, Skolithos. 5, Monocraterion. Cruziana ichnofacies: 1, Bergaueria. 2, Thalassinoides. 3, Phycodes. 4, Rosellia. 5, Asteriacites. 6, Arenicolites. 7, Curvolithus. 8, Lockeia. 9, Protovirgularia. 10, Teichichnus. 11, Rhizocorallium. Based on Buatois, Mángano, and Aceñolaza (2002).

Needless to say, bathymetry is a second-order control in onshore-offshore ichnofacies gradients. Onshore-offshore replacement models work only when changes in the other environmental factors parallel water depth. In the case of tidal shorelines, the Skolithos ichnofacies may occur in very shallow water where local environmental conditions are favorable, such as where high energy conditions occur in intertidal runoff channels (e.g., Weissbrod and Barthel, 1998).

Implications in Evolutionary Paleoecology


During the past decade, trace fossils have become increasingly important in our understanding of the evolution of benthic communities. Bambach (1983) interpreted the history of life as a process of colonization that implies the exploitation of empty or under-utilized ecospace. Trace fossils may provide crucial evidence for the recognition of spatial and temporal patterns and processes associated with benthic colonization. In particular, ichnologic information has been used in evolutionary paleoecology to help understand a number of problems, including paleoenvironmental trends of individual ichnogenera through time (Bottjer et al., 1988); paleocommunity behavioral evolution within particular biotopes (Seilacher, 1974, 1977b); colonization and diversification patterns (Crimes, 1974; Crimes and Crossley, 1991; Crimes and Droser, 1992; Crimes and Fedonkin, 1994); onshore origination and subsequent migration of complex behavioral strategies (Crimes and Anderson, 1985); trends in the extent and depth of bioturbation (Thayer, 1983; Droser and Bottjer, 1988, 1989, 1993; Bottjer and Droser, 1994); and colonization of nonmarine environments (Maples and Archer, 1989; Buatois and Mángano, 1993a; Buatois, Mángano, Genise, et al., 1998e).

Ichnologic studies have shown that after a rapid diversification of shallow-water trace fossils, complex behavioral strategies (represented by graphoglyptids and ornate grazing traces) dispersed into the deep sea (Crimes, 1974; Crimes and Anderson, 1985). Additionally, body-fossil data show onshore evolutionary innovations and subsequent offshore migrations (Sepkoski and Sheehan, 1983; Sepkoski and Miller, 1985). However, not very much is known about the precise depositional environment where most of the evolutionary innovations occurred. Analysis of tidal-flat ichnofaunas may shed some light on current problems in evolutionary paleoecology.

Tidal Flats as Sites of Evolutionary Innovations

As noted by Reise (1985), tidal flats are geologically ephemeral systems. Depending on the transgressions and regressions of the sea, tidal flats of a given geographic region rarely last longer than 104 years (Reise, 1985). In contrast to the long-term temporal instability, tidal flats are, on a daily basis, highly predictable systems governed by tidal cyclicity. Tidal flats usually are regarded as harsh, heterogeneous, physically controlled environments. From a biological perspective, tidal flats are highly heterogeneous, "open systems" where interspecific interactions are poorly regulated and open to numerous possibilities (Reise, 1985). Valentine (1976) related genetic variability in populations with physically controlled communities inhabiting unstable environments. In such a framework, selection will promote adaptive innovations. These ecologic attributes of tidal-flat communities, heterogeneity and unrefined interactions, together with predictability may have provided the appropriate ground for major steps in evolution (Reise, 1985).

Presence of large specimens of Lockeia siliquaria on the soles of Waverly lower-intertidal sandstones suggests that these traces were emplaced relatively deep below the sediment-water interface. As discussed previously, these burrows form palimpsest assemblages on time-averaged surfaces, which record repeated events of erosion, deposition, and recolonization. The presence of the bivalve Wilkingia, the most likely tracemaker of L. siliquaria at Waverly, reveals valuable information on evolutionary innovations (Mángano et al., 1998). Wilkingia may represent an evolutionary adaptation for siphon-feeding in the late Paleozoic, preceding the subsequent Mesozoic radiation of siphon-feeding infaunal bivalves (cf. Stanley, 1968, 1972). Stanley (1968) emphasized the role of mantle fusion and siphon formation as the key features that led to the Mesozoic infaunal bivalve radiation. Stanley (1972) argued that the virtual absence of deep burrowing bivalves in the Paleozoic was related to a non-siphonate condition and inefficient burrowing mechanisms. In extant rapid burrowers, siphons and ventral mantle fusion allow sealing of the mantle cavity, resulting in rapid foot extrusion and ejection of water that fluidizes the sediment around the shell (Trueman, 1966; Trueman et al., 1966; Stanley, 1970; Seilacher and Seilacher, 1994). Absence of ventral mantle fusion and true siphons (i.e., cylindrical tubes formed by fusion of the mantle edges at more than one point in the posterior region of the shell; Yonge, 1948, 1957) in late Paleozoic infaunal suspension feeders probably resulted in sluggish burrowers.

Wilkingia has an elongate shell and relatively deep pallial sinus, and it has been regarded as the first anomalodesmatan to be adapted to deep burrowing (Wilson, 1959; Runnegar, 1972). Whether or not Wilkingia was an efficient burrower that could cope with physical hazards of the coastal area is difficult to assess. In short, the presence of Wilkingia and associated relatively deep bivalve structures in this Carboniferous tidal flat may indicate incipient exploitation of the deep infaunal ecospace by bivalves, long before the Mesozoic revolution (Mángano et al., 1998).

Tidal-flat Ichnofaunas through Time

Comparison of the Waverly tidal-flat ichnofauna with other assemblages formed in similar environmental settings may be useful in addressing the problem of onshore replacement and offshore migration of benthic faunas through time. Bivalve trace fossils are, by far, the dominant biogenic structures in the Waverly tidal-flat deposits. Similar ichnofaunas have been recorded in other late Paleozoic tidal flats (e.g., Rindsberg, 1994). These ichnofaunas are remarkably different from those recorded in early Paleozoic and post-Paleozoic tidal flats. Early Paleozoic tidal-flat ichnofaunas are dominated by trilobite traces. For example, Mángano et al. (1996) described an ichnofauna from Late Cambrian-Early Ordovician intertidal facies in northwest Argentina that includes a wide variety and abundance of trilobite traces, comprising several ichnospecies of Cruziana, Rusophycus, and Monomorphichnus. Similar trilobite-dominated ichnofaunas have been documented in other early Paleozoic tidal flats (e.g., Baldwin, 1977; Legg, 1985; Durand, 1985; Astini et al., 2000; Mángano and Buatois, 2000; Mángano and Astini, 2000; Mángano et al., 2001).

These observations suggest that trilobite faunas were replaced by bivalve faunas on tidal flats during the mid-Paleozoic. Sepkoski and Miller (1985) noted a correspondence between evolutionary faunas and local marine communities, namely the Cambrian (trilobite-rich), Paleozoic (brachiopod-rich), and Modern (molluscan-rich) faunas. Temporal changes in environmental distribution of each of these communities display onshore-offshore expansions. Based on a Q-mode factor analysis of body-fossil communities, Sepkoski and Miller (1985; fig. 6) documented a replacement of trilobite-rich communities by mollusk-rich communities in shallow-water niches throughout the Paleozoic. Ichnologic analysis of Paleozoic tidal-flat ichnofaunas supports this model and suggests the importance of tidal flats as nurseries of evolutionary innovations. Bivalves, in contrast to articulate brachiopods, were particularly adaptable to physically unstable, stressful nearshore settings (Steele-Petrovic, 1979). Recent studies in the midcontinent of North America by Olszewski (1996) documented a striking ecologic segregation between articulate brachiopods and bivalves, and this may indicate a higher tolerance of bivalves to unstable environments.

Mesozoic and Cenozoic tidal-flat ichnofaunas are quite different from their Paleozoic equivalents, but they share many similarities with Holocene examples. For example, Cretaceous tidal-flat deposits of the Dakota Group in Colorado contain an abundant ichnofauna represented by deep to relatively deep burrows (Chamberlain, 1980; Mángano and Buatois, unpublished observations). Cenozoic tidal-flat deposits of Patagonia are dominated by deep crustacean burrows (Buatois, unpublished observations). Preliminary information suggests the importance of crustaceans and polychaetes as dominant elements of post-Paleozoic tidal-flat ecosystems. This seems to be the case in most modern tidal flats, where these groups dominate (e.g., Howard and Dörjes, 1972). For example, Curran and Harris (1996) estimated that the callianasid Glypturus acanthochirus could move 118.6 kg of sediment/m2/yr to the surface of a modern sand flat on San Salvador Island. Additionally, crustaceans and polychaetes produce large quantities of argillaceous fecal pellets, and they therefore are important agents of biosedimentation and modifiers of substrate properties (Pryor, 1975; Bromley, 1996). Establishment of crustacean communities in tidal-flat ecosystems may have played a significant role in the offshore expansion of bivalves during the Mesozoic.

Additionally, deep infaunal crustaceans are key bioturbators that commonly produce elite trace fossils (Bromley, 1990, 1996). Burrowing activities of crustaceans cause significant sediment reworking and obliteration of shallower tiers. Paleozoic tidal-flat ichnofaunas typically display a high diversity of shallow-tier trace fossils, and Mesozoic to Cenozoic ichnofaunas are biased towards deeper tiers and commonly exhibit only moderate levels of ichnodiversity. This trend likely represents a taphonomic artifact resulting from the dominance of deep infaunal crustaceans in post-Paleozoic tidal-flat ecosystems.

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Kansas Geological Survey, Geology
Placed on web May 21, 2015; originally published 2002.
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